Aromatic polymers have been conventionally used in various fields because of their high thermal stability and high dynamical stability as compared with those of aliphatic polymers. Of those, polyimide, polyamideimide, polyamide, polyetherimide, polysulfone, polyethersulfone, and the like each having an aromatic functional group are each particularly excellent in, for example, chemical resistance and mechanical properties. Accordingly, investigations have been conducted on the application of porous polymer films formed by turning those aromatic polymers into porous thin films to gas separation films and electrolyte membranes for fuel cells.
A method described in Japanese Patent Application Laid-Open No. 2007-169661 (in the case of polyimide) is available as a method of producing any such porous polymer film. To be specific, the method is a method involving: forming a coating film from a polyamic acid solution obtained by a polycondensation reaction between a tetracarboxylic dianhydride and an aromatic diamine; and immersing the film in a poor solvent for a polymer solution to perform solvent substitution so that the phase separation precipitation (formation of a porous body) of polyamic acid may be induced (hereinafter referred to as “phase separation method”). A polyimide porous polymer film can be produced by the imide conversion of the porous polymer film obtained by the method through a heat treatment.
In methods of producing the porous polymer films based on the phase separation method, the films are formed of, for example, polyimide and polyamideimide each having an aromatic functional group, and polymer solutions each having a high viscosity are used as raw materials. As a result, it has been difficult to control a porosity. This is because the porosity of a porous polymer film to be obtained strongly depends on the volume fraction of a solvent in a polymer solution serving as a raw material.
In addition, the shapes of voids formed by the phase separation method are determined by the substitution and diffusion of the poor solvent in the coating film, and hence a void diameter distribution often varies to a large extent. In view of the foregoing, a technology involving covering the coating film before a phase separation operation with a solvent substitution rate adjustor to reduce the variation has been disclosed (for example, Japanese Patent No. 4,110,669 and Japanese Patent Application Laid-Open No. 2003-138057).